1. Field of the Invention
The present invention relates to non-destructive testing equipment and more specifically, a method and apparatus for detecting rare earth metal oxides in the form of mold shell fragments in non-magnetic metal or metal alloy castings.
2. Description Related Art
Titanium castings for the aircraft industry today, as well as for other structurally demanding applications, require quality assurance inspection methods that locate as well as identify flaw or possible flaw indicator areas or conditions within them. Additionally these areas must be identified at some required minimum threshold level of detection with regard to both size and depth location within the part being tested. This demands the test method be sensitive to the particular type of flaw or flaw indicators that are inherent to a specific component, material of that component fabrication, or industrial process used in the parts manufacture such as, titanium and any non-magnetic titanium alloy, or other non-magnetic metal or metal alloy component or assembly formed by various casting processes. The present invention addresses the needs of industry for a more reliable and sensitive method for inspecting titanium, or other non-magnetic metal or alloy castings.
One insidious type of flaw that occurs in investment castings are mold shell fragments. These are inadvertently released from the ceramic shell mold during casting as a result of high thermal stresses and erosion of the mold by the molten metal. The ceramic mold innermost layer (that faces the molten metal) typically contains a rare earth metal oxide(s), for example, erbia. Rare earth oxides are utilized because of their high melting point and chemical compatibility with the reactive titanium melt. Upon release, the mold shell fragments are incorporated into the body of the casting itself, and thereby become inclusion defects. These defects reduce the ability of the part to carry design loads, and must be detected as extensively as possible to avoid compromising the reliability and efficiency of the system that the castings serve. Conventional techniques for detecting such defects have proved inadequate. The present invention hereinafter described addresses the need for an improved method of detection of this type of defect.
a. Prior Nondestructive Testing Methods
A variety of nondestructive methods have been developed to locate and identify castings defects, although these are principally confined to radiography, ultrasonic and eddy current techniques. Radiography is limited by poor contrast between the ceramic inclusions and the titanium alloy itself, since they exhibit similar x-ray penetration densities. This reduces the resolution of radiographic detection to unsatisfactory levels. Large section thicknesses also present difficulty due to high attenuation at available x-ray facilities. Ultrasonic methods suffer partially from the lack of a definitive interface (i.e., a large change in sound speed or wave character) at the defects, and partially from the irregular geometry of many of the structural castings currently being implemented. Eddy current techniques are limited to surface and near-surface defects, and thus of limited utility in the present situation where the inclusions may be deep within the casting body.
3. Prior Art Patents
U.S. Pat. No. 3,065,412 L. A. Rosenthal, Metal Detector
This patent discloses a device for detecting pieces of metal in a non-metallic material. However, this detects the metals by passing the substance between a pole of the magnet and a magnetic backing plate. It senses a change in magnetic flux, using a sensing coil, around the magnet. This system is intended for an assembly line production operation. In contrast, the present invention uses a permanent magnet but does not require a magnetic backing plate between the substance and the magnet. Additionally, the present system utilizes a Hall effect sensor to detect the change in magnetic flux.
U.S. Pat. No. 4,518,919, Ishida, Detecting Device for Detecting a Magnetic Strip Embedded in a Sheet
This patent discloses an approach for detecting a magnetic strip embedded in a sheet, such as a currency note. The method disclosed has a permanent magnet located above the conveyor of the currency, and a magnetic resistance detecting element (InSb) located below the currency. When the currency containing the magnetic strip is passed between the magnet and the detecting element, the change in magnetic flux is detected.
U.S. Pat. No. 5,105,151, Takahashi et al., Method and Apparatus for Magnetically Detecting a Carburized Portion of an Article While Discriminating a Non-Carbonized Deteriorated Layer of the Article.
U.S. Pat. No. 5,128,613, Takahashi, Method of Inspecting Magnetic Carborization in a Non-Permeable Material and Probe Therefore
Both of these patents disclose a method for detecting a carburized deposit located on the inside wall of a tube. The method of detection utilizes a magnet and a Hall sensor. However, in the first patent two magnets and two sensors are required in order to compare the flux change at the outer surface of the tube with the flux change at the inner surface of the tube. In the second patent, the Hall sensor is positioned between the poles of the magnet and the detecting surface of the Hall sensor is parallel to the direction of the undisturbed field lines. In contrast, the present invention uses a magnet and a Hall sensor to measure a change in the magnetic flux caused by a paramagnetic substance and only one magnet is needed. Additionally, in an embodiment of the present system the Hall sensor is placed directly below and to the side of one of the magnet poles and the detecting surface is perpendicular to the unperturbed field lines.
U.S. Pat. No. 5,432,444, Yasohama et al., Inspection Device Having Coaxial Induction and Exciting Coils Forming a Unitary Coil Unit
This patent discloses a method for inspecting an object for defects using an exciting coil for generating an electromagnetic field and an induction coil which are integrally connected to each other so as to induce mutual inductance in the induction coil by the electromagnetic field. This detection method is in contrast to the present method which does not sense a change in mutual inductance.
U.S. Pat. No. 5,589,772, Kugai, Non-Destructive Eddy Current Sensor Having a Squid Magnetic Sensor and Selectively Positionable Magnets
This patent discloses a method of detecting defects in objects. However, the detection method is comprised of a SQUID. A SQUID (Superconducting Quantum Interference Device) requires a liquid nitrogen cryogenic support system to provide the low temperatures needed to achieve superconductivity in the detector. The present invention does not require such a costly and complex system.
Further prior art references include:
U.S. Pat. No. 4,814,734, Moran, Search Coil Assembly for Metal Detectors;
U.S. Pat. No. 4,902,997, Moran, Search Coil Assembly for Metal Detectors; and
U.S. Pat. No. 4,943,770, Ashley-Rollman et al., Device for Accurately Detecting the Position of a Feromagnetic Material Inside Biological Tissue.